Fatigue can be a prevalent and serious problem for the individual with cancer and can negatively impact on the individual's quality of life. Little is known about the prevalence of clinical fatigue among patients with cancer and how the fatigue cancer patient's experience compares with the fatigue people experience as a function of their normal daily activities. This study, which utilized a control group, investigated the prevalence of fatigue among patients receiving treatment with radiotherapy (n = 54) and chemotherapy (n =47) over two measurement points. The level of fatigue experienced by cancer patients was compared with the level of fatigue experienced by apparently healthy auxiliary staff (n = 53) working at three cancer treatment facilities. There were no differences in the mean level of fatigue experienced by cancer patients and the mean level experienced by healthy controls before the start of cancer treatment. However, cancer patients experienced a significant increase in fatigue over a 5- or 6-week course of radiotherapy and 14 days after treatment with chemotherapy, and these increases were significantly greater than the fatigue reported by healthy control subjects. The midpoint of the Pearson Byars Fatigue Feeling Checklist was accepted as a crude measure of clinical fatigue and was found to be significantly different from the mean level of fatigue reported by healthy controls. The prevalence of fatigue among patients after undergoing cancer treatment was determined to be 61%. Fatigue in cancer patients was found to covary with weight, symptom distress, mood disturbance, and alterations in usual functional activities. The best predictors of fatigue in the patient sample were their symptom distress and mood disturbance. Symptom distress and fatigue were significant predictors of impairment in functional activities related to illness. Implications for practice and future research are discussed.